Zinc oxide (ZnO) is a typical Group II-VI semiconductor material and is a compound with a direct wide-band gap (3.37 eV). It has excellent optical and electrical properties, and great conditions for emitting blue light or near ultraviolet light. Thus, zinc oxide has the potential in the development of a variety of light emitting devices emitting ultraviolet light, green light, or blue light, etc. Semiconductor photoelectric devices made from zinc oxide mainly include: ultraviolet detector, light emitting device (LED), and semiconductor laser device (LD), etc. Furthermore, an inorganic piezoelectric thin film made from nano zinc oxide can be used to produce an ultra-thin bulk acoustic device. Moreover, a ZnO thin-film sensor has the following advantages: rapid response, high degree of integration, low power consumption, high sensitivity, good selectivity, and cheap raw material, etc.
Generally, there are two types of process in carrying out ZnG thin-film deposition: one is solution chemical deposition, and the other is vapor deposition. The solution chemical deposition process includes a sol-gel process and a metal-organic decomposition process. The vapor deposition process includes a physical vapor deposition (PVD) process and a chemical vapor deposition (CVD) process. A CVD process comprises vaporizing a compound containing zinc, and depositing ZnO on a substrate. However, the vaporization step requires a rather high temperature. Therefore, later on a different deposition technique is developed using an organic metal complex precursor as raw material for reducing the vaporization temperature. Such a deposition technique is called a Metal Organic Chemical Vapor Deposition (MOCVD) technique. A MOCVD can be used to deposit thin films for dielectric material, conductor, or semiconductor, and includes the following advantages: good control in atomic quantitative ratio among different elements, larger area size matching, good film uniformity, good smoothness in step coverage, and high deposition rate, etc.
U.S. Pat. No. 4,751,149 discloses a method for deposition of a zinc film in a relatively low temperature environment, and also discloses a process for reducing the resistance of a zinc film by adding various additives. This prior art invention uses an alkyl zinc as a CVD precursor to grow a ZnO thin-film, in which the deposition temperature is about 200° C. and the thin-film resistivity is 10−4˜10−2.
U.S. Pat. No. 6,416,814 discloses novel Sn and Zn ligand complexes, MXnL2, which can be used as CVD precursors. Said precursors have high reactive activities in a CVb process, while maintaining a high deposition film quality. Said MXnL2 are SnCl4L2 or ZnR2L2, wherein L is a ligand, e.g. methylformate, ethylformate, n-propylformate, n-butylformate, i-butylformate, t-butylformate or a mixture thereof; R is C1-8 alkyl or C2-8 alkenyl.
Metal ligand complex precursors for CVD usually use some bi-dentate ligands, e.g. acac (acetylacetone), thd (2,2,6,6-tetramethylheptane-3,5-dionate), thme (tris(hydroxymethyl)ethane), tdh (1,1,1,6,6,6-hexamethylheptane-2,4-dione), and fluoro-containing β-diketonate such as hfa (1,1,1,5,5,5-hexafluoro-2,4-pentanedione) and fod (2,2-diethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione). Fluoro contained in the ligand is for increasing the volatility of the precursor. However, many microelectronic processes do not permit the presence of halogen, such as fluorine. U.S. Pat. No. 6,099,903 discloses a novel ligand structure as shown in the following formula, wherein Y and Z separately are O, S or NR9:
wherein the definitions of R1, R2, R3, R4, R5, R9 and E1 can be found in said patent.
The disclosures in the above-mentioned US patents are incorporated herein by references.